Sodic Pyroxenes and Amphiboles from Koga Syenites of Ambela Granitic Complex, NWFP Pakistan
Abstract
The Koga syenites are intruded and fenitized by at least two corbonatite intrusions having different Na/K ratios. The carbonatite with high Na/K ratio formed Na-fenites, while the low Na/K ratio produced K-fenites.
Amongst the fenitic pyriboles, Na-pyroxene shows a trend from acmitic to diopsidic and/or hedenburgitic component in contrast to magmatic trend. The fenitic Na-amphibole occurs only in Na-fenitized rocks and shows a trend from magnesio-arfvedsonite to richterite which is opposite to the magmatic subsolidus trend. The amplibole composition shows an increase in MgO ratio with increasing intensity of fenitization and suggests the crystallization of fenitic amphibole under strong oxidation conditions.
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